A US soldier facing murder charges for allegedly killing Afghan civilians for sport has been linked to the 2004 deaths of several unarmed Iraqis, it has emerged.

Sgt Calvin Gibbs is accused, along with four other soldiers, of opening fire on Afghan civilians in unprovoked assaults between January and May in Kandahar province. Seven others are accused of dismembering bodies and removing bones.

The career of the alleged ringleader of a self-described "kill team" is now being scrutinised after he allegedly boasted to fellow soldiers of his exploits in Iraq, where he served two terms.

During interrogation, Sgt Gibbs revealed a tattoo on his left calf of a crossed pair of pistols framed by six skulls, which he told investigators was his way of keeping count of his victims, according to a report in the Washington Post, which cited army documents seen by the newspaper.

Sgt Gibbs allegedly told investigators that three of the skulls, coloured red, represented kills in Iraq, while the other three in blue were from Afghanistan.

Special agents from the US army's criminal investigations command are now re-examining an incident in 2004, when Sgt Gibbs and other soldiers allegedly fired on an Iraqi family in a car, killing two adults and a child.

The US army is understood to be searching for dozens of digital photos allegedly taken by soldiers showing their colleagues posing with Afghan civilian corpses. If released in public, they could create a worldwide furore similar to that sparked by the images of American guards mistreating Iraqi prisoners at Abu Ghraib, and would undermine the US effort to win over the Afghan public.

The war crimes investigation is the most serious the US army has faced in Afghanistan during its nine-year presence.

Soldiers who served with Sgt Gibbs in Afghanistan allegedly told investigators he pressed his comrades to cut fingers off Afghan corpses, and kept at least two fingers wrapped in cloth hidden in an empty water bottle.

Some allegedly claimed he planned to intimidate other members of his unit to keep quiet, and one soldier said Sgt Gibbs claimed he planned to make a necklace with the fingers.

He and four other soldiers are currently charged with conspiracy to murder three unarmed Afghans between January and May, though new records show a fourth may have been killed. Seven other soldiers are accused of dismembering bodies and removing bones.

Sgt Gibbs's lawyer has not commented this week, though he has previously said that the killings were combat-related.

The first hearing into the deaths opened on Monday in Washington state, where the brigade is based, but was delayed after several witnesses and three of the accused invoked their right to remain silent.

According to Cpl Emmit Quintal, drug use was rampant in Sgt Gibbs's unit. He said that some members of his platoon "had been smoking hashish consistently... sometimes as often as every day or every other day".

The drug was allegedly purchased from their Afghan interpreters or truck drivers passing through Forward Operating Base Ramrod in Kandahar province.